Why does my commputer cpu shocks

Paperdoc

Polypheme
Ambassador
Important question: do you get one quick shock when you touch the case, then nothing more as you hold on? Or, do you get a continuous tingling as you hold it?

A single shock at first is most likely to be a static electric discharge. If this is the case, the good news is your computer's case is grounded as it should be. The bad news is you are somehow building up electric charge and some day that will zap an important component and damage it. Find out why you have so much static charge being generated.

If you get a continuous shock as you hang on, there is a serious problem with power connections between the computer, the wall power supply, and some of the peripheral devices plugged into the computer. If this is the case, let us know becasue ther are several steps yous should take to eliminate it.
 
Also google "floating ground".

If you get a continuous tingle, your case, and therefore your PSU, is not properly grounded.

Things to check:
Defective PSU line cord. Try another one.
Defective power outlet. Try another one outlet in a different room.
PSU improperly mounted. This is unlikely because the PSU is screwed to the case.